How the easier way to calculate short circuit rating (kA) for the AC?

It is not necessarily easy to calculate the short circuit rating (fault rating) for any type of equipment. That is normally a parameter that the equipment vendor guarantees, or that the design engineer specifies for equipment to install in a given system.
This rating shall generally be better than the fault level at the installation site. The fault level can be calculated from the supply voltage and the related source impedances (source in parallel with the load).

I0"=Un/((sqrt(3))*Ze");
I"=1,1*Un/((sqrt(3))*Ze");
I'=1,15*Un/((sqrt(3))*Ze');

I0"-short circuit current in subtransient period (generators are under no-load conditions)
Un-rated voltage
Ze"-equivalent impedance of power system at the place of fault (subtransient period)
I"-short circuit current in subtransient period (generators are under full-load conditions)
I'-short circuit current in transient period
Ze'-equivalent impedance of power system at the place of fault (transient period)

Bojan,
What you correctly calculate here is the fault level.
Maybe that is what Mohd referred to when asking for "short circuit rating"?

Are you asking for the MVA which is equivalent to fault level or are you just asking for the fault current. This indicates the maximum fault stress that the network can reach during fault condition.